Explore the Transient X-ray Sky with Einstein Probe
Weimin Yuan1 , and the Einstein Probe team
1) Key Laboratory of Space Astronomy and Technology, National Astronomical Observatories,
Chinese Academy of Sciences, Beijing 100012 China
E-mail(WY): wmy @ nao.cas.cn
Abstract
The Einstein Probe is a small mission of the Chinese Academy of Sciences dedicated to time-domain
astrophysics to monitor the sky in the soft X-ray band. It will carry out systematic surveys and characterisation of high-energy transients at unprecedented sensitivity, spatial resolution, Grasp and monitoringcadence. Its wide-field imaging capability is achieved by using established technology of micro-pore lobstereye X-ray focusing optics. Complementary to this feature is deep X-ray follow-up capability enabled bya narrow-field X-ray telescope with a larger eﬀective area. It is also capable of real time triggering anddownlink of transient alerts on the fly, in order to activate multi-wavelength follow-up observations byother astronomical facilities worldwide. Its scientific goals are concerned with discovering new or raretypes of transients, particularly tidal disruption events, supernova shock breakouts, high-redshift gammaray bursts, and possible X-ray sources associated with gravitational wave events. The mission is approved
and fully funded, aiming for launch by the end of 2022 with a nominal lifetime of 3 years.
Key words: X-ray: instruments — X-ray: time-domain astronomy — mission: Einstein Probe

1.

Introduction

The X-ray sky is rich in transients and variables of various types. With diverse timescales from sub-seconds toyears, a large variety of such dramatic objects have been
discovered and extensively studied ever since the earlydays of X-ray astronomy, thanks to successive all-sky
monitors in the X-ray (as well as γ-ray) waveband. In
recent years the successful operations of Swift (Gehrels et
al. 2004) and MAXI (Matsuoka et al. 2009) have greatly
expanded our horizon in monitoring the X-ray sky andadvanced our knowledge about the dynamic Universe(Gehrels & Cannizzo 2015; Mihara et al. 2017). New
transients, particularly of previously unknown and scientifically important types, continue to be discovered.
These include new Galactic black hole X-ray transients(candidates), relativistic outbursts on long timescales interpreted as arising from jets launched by massive black
holes following tidal disruption events, gamma-ray bursts(GRBs) at high redshifts up to z ∼ 9, supernova shock
breakouts, etc. To characterise and understand thesenew phenomena, a large sample of events and detailed
observations are needed.
With the advent of major wide-field sky-monitoring facilities across the entire electromagnetic spectrum, such
as LSST and LOFAR/SKA, the turn of the decade will
see a golden age of time-domain astronomy with flourishing discoveries, as generally anticipated. Even more
excitingly, gravitational-wave (GW) astronomy will ma-ture with the 2nd generation of GW detectors being able
to detect a potentially large number of events. They callfor synergy observations across the electromagnetic-wave
spectrum to detect their potential electromagnetic counterparts. This is essential for identifying the associatedobjects/galaxies and to try to comprehend the nature of
GW events.
In the X-ray regieme, the above driving science invokes
the next generation of instruments with higher sensitivity and improved angular resolution (a few arc-minutes
or better) than those currently available. These requirements are now readily enabled by novel X-ray focusing
optics—the lobster-eye micro-pore optics, in which X-ray
focusing imaging results in enormously enhanced gain insignal to noise, and thus high sensitivity, while a wide
Field-of-View (FoV) can be maintained.
The Einstein Probe (EP) (Yuan et al. 2016b) is an approved and fully funded space astronomy mission of theChinese Academy of Sciences (CAS), selected for implementation in the 2nd phase of its Space Science Programme. The mission is scheduled for launch by the endof 2022, with an operation lifetime of three years and five
years as a goal. EP’s wide-field capability is achieved bymaking use of the lobster-eye focusing technology, withan aim to monitor a large fraction of the sky at highsensitivity and cadence in the soft X-ray band.

2. Scientific objectives
Einstein Probe will carry out systematic wide-fieldsky monitoring surveys in the soft X-ray band withsensitivity one order of magnitude higher than thosecurrently in orbit. The primary science objectives are:(1) Discover and characterise cosmic X-ray transients,
to reveal their properties and gain insight into theirnature and underlying physics.
(2) Discover and characterise X-ray outbursts fromnormally quiescent black holes, for better understandingof the demography of black holes and their origin andevolution, as well as accretion physics.
(3) Search for X-ray sources associated withgravitational-wave events and precisely locate them.
The mission will address some of the key questions inastrophysics and cosmology, such as the prevalence of
black holes in the Universe and how they formed andevolved, and how black holes accrete mass and launch
jets; the astrophysical origins and underlying processesof gravitational wave events; details of the physics which
operates in extreme conditions of strong gravity; when
and where did the first stars form in the early Universeand how they re-ionise the Universe; the progenitors andprocesses of supernovae (Yuan et al. 2016a).
2.1. Systematic census of transients in the soft X-ray sky
EP will carry out systematic surveys to discover andcharacterise high-energy transients of various types overa wide range of timescales and at high cadence in thesoft X-ray band. It will also perform immediate followup observations of newly discovered transients with its
narrow-field X-ray telescope onboard, and will issue fastalerts to trigger follow-up observations by global multiwavelength facilities. Most of the previous and current
high-energy astrophysical monitors are operating in themedium and hard X-ray (even gamma-ray) bands, andthe soft X-ray sky (from a fraction of 1 keV to several
keV) has hardly been monitored extensively with both
suﬃcient sensitivity and cadence. It is thus expectedthat EP will open up a new window in time-domain Xray astronomy. This implies that the bulk of the targets
and processes being monitored will probably change fromthe non-thermal, highly relativistic beaming regime (e.g.
GRBs, blazars) to thermal, low-/non-beaming regime(e.g. accretion, shock), and thus new types of transientsare expected to emerge. Above all, its increased sensitivity will enable unprecedented census of faint or distanthigh-energy transients beyond the horizons of the previous and contemporary X-ray monitors. As such, a large
variety of X-ray transients are expected to be detected,
even possibly previously unknown types.
Of particular interest, EP is expected to discover alarge sample of black hole tidal disruption events, outnumbering the sparse and heterogeneous cases currently
known, and may revolutionise the research of the field(see next subsection). Other important types of transients include GRBs at higher redshifts than the currently known sample (7 GRBs with z=6.2–9.4). These
valued events, produced by violent death of stars in theearly Universe, would carry unique information on earlystar-formation and metallicity evolution, and hopefullyon the first generation of stars and the re-ionization in
the dark early Universe, which are otherwise almost inaccessible from ground-based facilities (Yuan et al. 2016b).
Shock breakout emission from supernovae is the prompt
X-ray emission produced as the outward-propagating
shocks generated by core-collapse breaks out of the stellar surface. X-ray observations can yield important clues
to the properties of the progenitor stars. There are only
a few candidate events known so far due to their elusiveness given the short burst durations (∼ 103 s) andmoderate brightness. EP is also expected to detect moreof such events. Other transient sources to detect andcharacterise in large numbers include X-ray flashes, lowluminosity GRBs, X-ray rich GRBs, GRB precursors,
magnetars, stellar corona flares, classical novae, supergiant fast X-ray transients, and outbursts of active galactic nuclei and blazars, etc.
2.2. Tidal disruption events and quiescent black holes
Stars are tidally disrupted and accreted when their selfgravity cannot balance the strong tidal force as they approach massive black holes (MBHs) closely enough, producing a flare of electromagnetic radiation, preferably
peaked in the UV and soft X-ray regime as predictedin theoretical models (Rees 1988). First discovered in
the ROSAT survey (Komossa & Bade 1999), there are
only a few dozen TDE candidates found so far, mostly inthe declining phase via searches from multi-wavelengtharchives and surveys (Komossa 2015). As perhaps themost unique signature of the existence of MBHs in otherwise quiescent galactic nuclei, a systematic survey ofTDEs provides a census of quiescent MBHs in the Universe, i.e. to constrain their occupation fractions in various types and masses of galaxies, which is essential forunderstanding the formation and evolution of massive
black holes and of galaxies. Recent discoveries of a few
jetted events by Swift and MAXI indicated that relativistic jets can be launched in at least some of the TDEs
(Burrows et al. 2011). A distinctive X-ray lightcurvewith large drops and recurrences was also seen in one
case, which was interpreted as perhaps the most accessible signature of quiescent binary MBHs, via tidally disrupting a star (Liu et al. 2014).
With its soft X-ray bandpass and high sensitivity, EP
is an ideal instrument to discover and study TDEs ina systematic way, and to catch them at the peaks of
their X-ray flares out to at least a few hundred Mpc.
The estimated detection rate of TDEs may range fromseveral tens to hundreds per year, depending on the ac-tual event rate per galaxy and the occupation fractionof MBHs. The resulting homogeneous sample of TDEs
obtained by EP in several years’ operation will provide
useful constraints on the galactic occupation fraction ofMBHs, by combining the incidence of TDEs for various
type of galaxies, which can be determined from simulations. Some of the TDEs may be caught at the risingphase of the flares, making it possible to observe at multiple wavelengths over the entire cycle of TDEs from theonset of disruption till complete fading away.
Moreover, EP is expected to discover new stellar- andintermediate-mass black holes lurking in our Milky Way
and in nearby galaxies, by detecting their X-ray outbursts due to some kind of instability of gas accretion.
In this sense, EP is a black hole finder at almost all astrophysical mass scales.
2.3.

Detecting electromagnetic sources of gravitationalwave events
Gravitational wave (GW) astronomy has come of dawn
with the direct detections of the first GW event ever,
GW150914 (Abbott et al. 2016) and subsequent a few
others. Up on the completion of upgrade of Advanced
LIGO, Advanced Virgo and other GW observatories, it
is highly anticipated that, at the turn of the decade, GW
events will be detected in large numbers. The detectionof their electromagnetic (EM) counterparts is of greatscientific significance in the sense that astrophysical observations in the EM domain will provide independent
evidence for further establishing the model interpretation of the events solely based on the general theory
of Relativity. Moreover, as the loci of GW sources canonly be determined within regions from several hundreds
to several tens of square degrees even for the upgradedglobal GW detector network, the detection of EM counterparts is essential for precisely locating the GW eventsand thus the identification of associated astrophysicalobjects. This will enable precisely measuring redshifts(thus the source distance and EM energy budget) andhelping understanding the nature, origin and evolutionof the progenitor systems. As a potentially even more
far-reaching cosmological application, joint detections of
GW and EM sources (redshifts) can be used to probe
the geometry and expansion of the Universe, and to determine the cosmological parameters (Schutz 1986).
Although no EM counterparts have been reported forthe several GW detections made so far1 , strong EM radiation associated with mergers of binary neutron stars
or neutron star–black hole binary has been predicted*1 Note added after publish: On Aug 17, 2017 a historicalgravitational-wave event GW170817 resulting from a mergerof two neutron stars was detected by ALIGO/AVIRGO and,remarkably, its electromagnetic-wave counterpart was also detected in multi-wavelength from gamma-ray to radio.

Fig. 1. Illustration of the light path of a point-like X-ray source
through a lobster-eye optics(Zhang 2013). With the combined high sensitivity, large
field of view, manoeuvrability and rapid alert downlink,
EP has great potential in detecting the EM counterparts of GW events by synergy with the next generationof GW detectors. This may be achieved either by directly detecting X-ray transient sources associated with
GW events that happen to be within the large EP FoV,
and/or by rapidly observing the sky region to cover thelarge loci area provided by the GW detectors as ToO observations. For the latter, fast uplink of GW alert data
is essential.
3. Lobster-eye micro-pore optics
In general, X-ray focusing is realised by multiple grazing incidence reflections on surfaces almost parallel tothe direction of incident X-rays. The conventionally andcommonly adopted configuration is the Wolter-I optics,
which reflects X-rays by a tubular, rotationally symmetric parabolic surface followed by a hyperbolic surface. The FoV of such optics is inherently small, typically less than one degree. Alternatively, the Lobster-eye
Micro-Pore Optics (MPO) (Angel 1979), which mimics
the imaging principle of the eyes of lobsters (Figure 1),
can reach an unrestricted FoV. The lobster-eye optics ismade of a thin spherical micro-channel plate with millions of square micro-pores, the axes of which all point
radially to a common centre of curvature. Incoming lightat a grazing incidence angle is reflected oﬀ the walls of
the many tiny pores, and is brought onto a focal spherewith a radius of half the curvature of the optics. For any
given direction of incoming light, a same large numberof pores, whose reflection surfaces are configured orthogonal to each other, take part in the reflection. Such aconfiguration has no preferential optical axis, and canthus deliver an un-vignetted FoV as large as, in principle, the entire solid angle of 4π.
Such a focusing optics produces true imaging witha cruciform point spread function, which has a central
peaking spot produced by two reflections from adjacentFig. 2. A demonstration prototype of a lobster-eye MPO telescopeassembly developed at X-ray Imaging Lab, NAOC (left-hand), anda true X-ray image formed on its focal plane for a point-like source(right-hand) showing the characteristic cruciform PSF of the lobster-eye optics

walls of the pores and cross-arms produced by single reflections. It can yield a large focusing gain of ∼2000,
and a moderate resolution of FWHM the order of arcminutes delivered by the currently commercially available MPO pieces. In practice, the MPO Lobster-eye optics can achieve a FoV of thousands of square degrees
with very light weight, which is unique for wide-field Xray imaging. Figure 2 shows a demonstration prototype
of a portion of the optics assembly of one WXT module(left panel), as well as an X-ray image obtained at the
focal plane for a point-like X-ray source, i.e. PSF (rightpanel)
4. Mission concept
4.1. Instruments
EP carries two scientific instruments—an X-ray monitoring instrument Wide-field X-ray Telescope (WXT) witha large instantaneous FoV, and a narrow-field (∼ 30′ )Follow-up X-ray Telescope (FXT). To achieve both wide
FoV and X-ray focusing, the novel micro-pore optics
in the lobster-eye configuration (see above) is adopted
for WXT. WXT consists of 12 identical modules with a
375 mm focal length, each of which is composed of 6 × 6
mosaicking MPO plates, subtending ∼ 300 square degrees. Figure 3 (upper panel) shows the layout of one
WXT module. An optical baﬄe is attached at the front
end of the MPO assembly to shield optical stray lightfrom the Sun, the Moon and the Earth. The 12 modules make a total vignetting-free FoV of WXT of ∼ 3600
square degrees (∼1.1 steradian), as illustrated in Figure 3(lower panel).
WXT has a large-format focal plane of approximately
420 mm×420 mm. The baseline choice of the focal plane
detectors is CMOS imaging sensors, developed at the
Gpixel Inc. in China. The current design is to mosaic the
focal plane detector of one WXT module by 2x2 CMOS

Fig. 3. Upper panel: Design of one module of the wide-field X-ray
telescope. Lower panel: Illustration of the field of views of the
WXT modules and the follow-up X-ray telescope

sensors. This new type of detectors, which are currently
being under development, have 4 k×4 k pixels (of pixel
size 15 micro) and 6 cm×6 cm in size. The CMOS detectors have some advantages over CCDs for their fast readout speed, as fast as several tens of frames per seconds.
They can thus be operated at moderately low temperatures. One WXT module weights 17 kg including the
MPO mirror assembly, detector and electronics unit, optical baﬄe, structure and thermal control.
The nominal detection bandpass of WXT is 0.5–
4.0 keV. Figure 4 shows the eﬀective area curves of WXT
derived via realistic ray-tracing simulations taking into
account the imperfectness of the MPO arrays (Zhao et
al. 2014). The focal plane detector is a CMOS sensor
coated with 200 nm-thick Aluminum. The peak eﬀective
area is ∼3 cm2 at around ∼1 keV for almost any direction
within the FoV except at the edge of the MPO plates.
Thought the eﬀective area for a given direction is nothigh, it has nearly the same value across the entire FoV,
yielding a very large Grasp (eﬀective area times FoV)of the order of ∼ 104 cm2 sq.deg., which is the largest
among all focusing instruments in X-rays. Based on
simulations of the X-ray background on the detector, including particle background and incident diﬀuse X-ray
emission (both the cosmic X-ray background and Galac-

Table 1. Specifications of WXT and FXTParameters
Number of modules
Field-of-view
Spatial resolu. fwhm
Bandpass (keV)
E resolution (eV) @1 keV
Eﬀective area @1 keV (cm2 )

WXT

FXT

12
3600 sq.deg.
5’
0.5–4.0170∼3

138’ (diameter)<2’
0.3–8.0
170≥120

lobster-eye modules to act as a soft X-ray transient monitor and a narrow-field telescope for deep follow-up observations gives unprecedented Grasp and sensitivity thatwill revolutionise X-ray time-domain astronomy.
Fig. 4. Simulated eﬀective area curves of WXT for the central
focal spot and plus the cruciform arms. The MPO arrays arecoated with Iridium, and have surface roughness of 0.55 nm
and the tilts of pores following a Gaussian distribution with
σ=0.85 arcmin. The focal plane detector is a CMOS sensor coatedwith 200 nm-thick Aluminum. For details of the simulations see
Zhao et al. (2017)

tic diﬀuse emission), we estimate a theoretical detecting
sensitivity for a point-like source2 for WXT to be approximately a few times 10−11 ergs s−1 cm−2 (in the 0.5-4 keVband) at a 1000 second exposure3 , much more sensitive
than previous and current wide-field X-ray monitors, including MAXI and Swift/BAT. The angular resolutionis ∼5 arcmin (FWHM) for the central focal spot, limited by the manufacturing precision of the MPO plates
for the current technology, and possibly by misalignmentand distortion of the plates introduced in the process ofassembling.
The EP FXT is composed of a focusing mirror assembly, which is of the conventional Wolter-I type with a
focal length of 1.6 m and a CCD detector at the focal
plane, cooled down to a temperature in the range of 70◦ – -100◦ C . Its covers an energy passband of 0.3–8 keVand has an eﬀective area of ≥ 120 cm2 at around 1 keV,much larger than that of WXT. The total weight of the
FXT module is about 146 kg.
The specifications of WXT and FXT are summarisedin Table 1. The combination of an array of wide-field*2 Assuming a power law spectrum with a photon index of -2 anda Galactic absorption column 3 × 1020 cm−2 .
*3 It should be noted, however, that these values are based on
the simulations and may be considered to be nominal; thetrue sensitivity may be somewhat lower considering possible
degradation of the performance of the actual MPO plates used,
as well as the actual environment of charged particles in orbit.

4.2. Mission Profile
One possible configuration of the EP payload is shown in
Figure 5. The payload has a weight of 340 kg and powerof 430 W, and whole satellite has a wight of ∼1050 kg
and power of ∼825 W on average. The satellite platform
is to be provided by MicroSat, CAS.
The satellite will be in a circular orbit at an altitudeof 550-600 km and a period of ∼97 minutes, and an inclination angle of ∼29◦ . The survey strategy of EP will
be composed of a series of pointings to mosaic the night
sky in the directions avoiding the Sun, with an avoidance angle to the edge of the WXT FoV always greaterthan 90 degrees, as illustrated in Figure 6. During its 97minute orbit of the satellite, three fields will be observedwith WXT on the night-side of the sky, each with a 15–
20 min exposure. Over three successive orbits almost theentire night sky will be covered, with sampling cadences
for a given sky region ranging from 5 to 25 times perday. Monitoring the night sky also makes it possible toperform follow-up observations of detected transients byground-based optical/IR telescopes. The set of pointing
directions is shifted by about 1 degree per day to compensate the daily movement of the Sun on the sky. In thisway, the entire sky will be covered within half-a-year’s
operation of the mission.
There are three basic observing modes for EP: themonitoring survey mode using WXT, the following observation mode using mainly FXT (while WXT continues to be in data acquisition mode), as well as the ToO
observation mode of both WXT and FXT upon requests
from the ground segment via the command data uplinkroute.
During observations of WXT, onboard computers will
search for transients from X-ray photon events collected
in real time over a range of timescales. Once a transient
source is detected, classified, and triggered by the data

Fig. 5. A possible configuration of the EP payload, with twelve WXT
modules and one FXT module (at the centre) (credit: MicroSAT,
CAS).

Fig. 6. Illustration of the field-of-view and staring observations in one
orbit (credit: MicroSAT, CAS).

Space Science Centre (NSSC).

processing and alerting system onboard, the satellite will
slew (with a slewing speed of 15 degrees per minute) toa new position to enable pointed follow-up observations
with FXT by targeting the new source within its FoV.
Meanwhile, WXT continues to monitor the new sky region centering the position of the transient. It is essential
to transmit alerts quickly to the ground segment, preferably within a few minutes or so. One of the designs is
to make use of the short text message capability of theChinese satellite navigation system Beidou, which covers
the Asia-Pacific region currently and will be extended tocomplete the global coverage by 2020. One interesting
feature of the Beidou system is its quick uplink capability, which will enable fast ToO observations, as requiredby follow-up observations of urgent multi-waveband andmulti-messenger transient sources such as GW events. A
second design is to make use of the French VHF groundstation system, which is being built for the joint ChineseFrench GRB mission SVOM, via international collaboration with CNES currently being under discussion. The
normal telemetry data will be transmitted to the groundsegment via the X-band using the ground tracking stations of CAS at Sanya, Kashi and Miyun in China.
The EP payload is designed and built jointly by a consortium of CAS’s institutions, mainly National Astronomical Observatories, Institute of High Energy Physics,
Shanghai institute of Technical Physics, and Shanghai
Micro-Satellite Engineering Center, etc. The Space Science Programme of CAS is managed by the National

The authors would like to thank the conference organisers for invitation to this interesting meeting. Gratefulness is given to the members of the Einstein Probe team,
who have had helped providing the materials and content of this paper. The work is supported by the Strategic Priority Research Program of the Chinese Academyof Sciences, Grant No. XDB23040100.
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